In certain industrial applications, such as the production of fiberglass wool for preparing thermal or acoustical insulation products, there is a need for metal alloys which possess certain characteristics such as high rupture strength, high corrosion resistance, and high oxidation resistance at elevated temperatures. In the fiberglass and mineral fibers industries, filaments which may be used for preparing such insulation products are produced by passing molten glass or mineral through the bores of the foraminous wall of a chamber which is adapted for rotation at high angular velocities (said chamber being known as a spinner). The filaments are extruded through the fiberizing bores due to the centrifugal action of the rotating spinner. Such spinners are typically operated at a temperature of about 2,050.degree. F. and an angular velocity of about 2,050 RPM. It is advantageous, from a fiberglass production cost standpoint, for the angular velocity to be as high as possible so as to maximize the rate at which filaments are extruded through the fiberizing bores. However, high spinner angular velocities result in reduced spinner life due to the limited corrosion resistance of typical spinner base metal alloys. The flow of molten glass or mineral through the spinner bores corrodes the cylindrical walls of the bores causing them to enlarge, resulting in a loss of control of fiber diameter.
It is well-known in the fiberglass manufacturing art to apply a layer of platinum or other precious metal or alloy over the surfaces of base metal components which contact the molten glass, to reduce corrosion of the base metal alloy by the glass. However, when platinum is intimately bonded directly to the base metal alloy of, for example, a fiberglass spinner, an undesirable interdiffusion zone of complex intermetallic compounds forms near the interface between the precious metal layer and base metal alloy. This interdiffusion zone becomes brittle and porous, causing the precious metal layer to fail.
The bores of a fiberglass spinner are especially susceptible to corrosion caused by molten glass. Johnson, D.C. "Platinum Coating Technique Developed For The Glass Industry," Glass, September, 1987, page 372 discloses a method for applying a platinum coating to the surfaces of fiberglass spinner bores. The platinum is bonded directly to the spinner base metal alloy. However, it has been determined that such a platinum layer results in the interdiffusion phenomenon described hereinabove. The present inventor has discovered that base metal alloy atoms diffuse into the platinum layer leaving a porous base metal alloy structure immediately surrounding the annularly-shape platinum metal layer. Eventually, the bond between the base metal alloy and platinum layer fails, causing the platinum layer to sluff away from the cylindrical surface of the spinner bore.
U.S. Pat. No. 4,404,009 to Bhatti et al. discloses a fiberglass spinner having precious metal tubular inserts which are press-fit into the spinner bores. Such inserts could experience the same interdiffusion failure described above.
British Patent Specification No. 1,190,266 to Selman discloses a refractory metal spinning die coated with a layer of boron nitride or silicon nitride and overlaid with a platinum sheath. The platinum sheath is applied by fitting suitably shaped pieces of platinum over the article to be enclosed and then welding these pieces together so as to form a closely fitting sheath or covering for the base metal core. Such a method could not, however, be used to coat the bores of a fiberglass spinner.
U.S. Pat. No. 4,721,518 to Monji et al. discloses a press mold for forming glass optical lenses. The mold comprises a base metal alloy mold surface conforming to the desired lens shape, a barrier layer of titanium nitride applied by chemical vapor deposition, and a glass-contacting noble metal layer applied over the barrier layer by sputtering. However, the disclosed process employing sputtering could not be used to coat the cylindrical surfaces of fiberglass spinner bores which have diameters on the order of less than 1 millimeter.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.
It would be desirable to coat the bores of a alloy metal base fiberglass spinner with a durable, corrosion-resistant layer of platinum or platinum alloy.